The present invention relates to a compressor used in an on-vehicle air conditioner. Particularly, the present invention pertains to a variable displacement compressor that varies its displacement based on environmental conditions.
FIG. 10 illustrates a typical variable displacement compressor. The compressor of FIG. 10 includes a front housing 201, a cylinder block 202 and a crank chamber 203, which is defined between the front housing 201 and the cylinder block 202. A drive shaft 204 extends and is rotatably supported in the crank chamber 203.
A cam plate, or swash plate 205, is supported by the drive shaft 204 in the crank chamber 203 by a lug plate 205a. The swash plate 205 rotates integrally with and is inclined relative to the drive shaft 204. The lug plate 205a is secured to the drive shaft 204 to transmit rotation of the drive shaft to the swash plate 205. The lug plate 205a is supported by a thrust bearing 205b located between the lug plate 205a and the front housing 201. A lip seal 203a is located between the circumferential surface of the front portion of the drive shaft 204 and the inner surface of an opening 201a of the front housing 201 to seal the crank chamber 203.
Cylinder bores 202a are formed in the cylinder block 202. A piston 206 is reciprocally housed in each bore 202a. The pistons 206 are coupled to the swash plate 205. A rear housing 208 is secured to the cylinder block 202 by way of a valve plate 207. A suction chamber 209 and a discharge chamber 210 are defined in the rear housing 208. Refrigerant gas is drawn into the suction chamber 209 before being compressed by reciprocation of the pistons 206 in the cylinder bores 202a. The compressed gas is then conducted to the discharge chamber 210.
A shaft bore 202b is formed in the center of the cylinder block 202. The rear portion of the drive shaft 204 is fitted in the shaft bore 202b. A snap ring 211 is fixed to the rear portion of the shaft bore 202b. A thrust bearing 212 is located at the rear end of the drive shaft 204. A coil spring, or support spring 213, is located between the thrust bearing 212 and the snap ring 211. The support spring 213 urges the drive shaft 204 forward and compensates for dimensional errors of the parts. The support spring 213 also prevents the drive shaft 204 from chattering in the axial direction. The front side of the drive shaft 204 refers to the end connected to a drive source, or engine Eg, and the rear end of the drive shaft 204 refers to the opposite end.
The discharge chamber 210 and the crank chamber 203 are connected by a supply passage 214. A control valve 215 is located in the supply passage 214 to adjust the flow rate of refrigerant gas. The control valve 215, which is an electromagnetic valve, controls the size of an opening between a valve body 216 and a valve hole 217 based on external information such as the temperature of an evaporator connected to the compressor, the temperature of the passenger compartment, a target value of the compartment temperature and the speed of the engine Eg. Accordingly, the difference between the pressure Pc in the crank chamber 203 and the pressure in the cylinder bores 202a is changed. The inclination of the swash plate 205 is changed in accordance with the changed pressure difference. The abutment of the swash plate 205 against a limit member or, stop ring 218, prevents the inclination of the swash plate 205 from being less than a predetermined minimum inclination.
An electromagnetic clutch 219 is attached to the front end of the drive shaft 204 to selectively transmit the force of the engine Eg. The clutch 219 includes an armature 220 and a pulley 221. The armature 220 is secured to the drive shaft 204 and includes a surface perpendicular to the axis of the drive shaft 204. The pulley 221 is coupled to the engine Eg. The armature 220 is located in front of the pulley 221. A core 222 is located next to the pulley 221. The armature 220 is coupled to and separated from the pulley 221 by exciting and de-exciting the core 222.
When the target compartment temperature is significantly changed in a short time, or when the engine speed is suddenly increased, the compressor displacement is minimized. At this time, the control valve 215 abruptly widens the opening between the valve body 216 and the valve hole 217 based on the external information. Accordingly, highly pressurized refrigerant gas in the discharge chamber 210 is suddenly conducted to the crank chamber 203, which quickly increases the pressure Pc of the crank chamber 203. In this case, the pressure difference between the crank chamber 203 and the cylinder bores 202a with the pistons 206 in between is suddenly increased. A sudden change of pressure dramatically decreases the inclination of the swash plate 205, which presses the swash plate 205 against the ring 218.
The thrust load acting on the drive shaft 204 will now be described. The force F acting on the drive shaft 204 is expressed by the following equation (1).                     F        =                  Fgh          -          Fsp          -                                    ∑                              i                =                1                            N                        ⁢                          S              ⁡                              (                                                      Pb                    ⁡                                          (                      i                      )                                                        -                  Pc                                )                                                                        (        1        )            
Fgh represents the force that the clutch 219 applies to the drive shaft. Fsp represents a load at the rear end of the drive shaft 204. N represents the number of the cylinder bores 202a. S represents the cross-sectional area of each cylinder bore 202a. Pb(i) represents the pressure in each cylinder bore 202a. Pc represents the pressure of the crank chamber 203. The equation (1) can be approximated by an equation (2) below, which has been obtained through experiments.                     F        =                  Fgh          -          Fsp          -                                    SN              7                        ⁢                          (                                                3                  ⁢                  Pd                                +                                  4                  ⁢                  Ps                                -                                  7                  ⁢                  Pc                                            )                                                          (        2        )            
Ps represents the pressure of the suction chamber 209 (suction pressure). Pd represents the pressure of the discharge chamber 210.
When the swash plate 205 is pressed against the stop ring 218, the equation (2), or the value F, is greater than zero (F greater than 0). In other words, the drive shaft 204 receives a rearward force. The rearward force acts as a compression load and is transmitted to the support spring 213 via the thrust bearing 212 thereby compressing the spring 213.
However, since the spring 213 is a coil spring, a change of the axial dimension of the spring 213, as shown in FIG. 4, does not significantly increases the force of the spring 213. Therefore, the support spring 213 allows the drive shaft 204 to move rearward. When the drive shaft 204 is moved rearward, the stroke range of the pistons 206, which are coupled to the drive shaft 204 through the swash plate 205, is moved rearward. Accordingly, the top dead center position of each piston 206 is moved rearward.
When each piston 206 is at the top dead center, a predetermined space exists between the piston 206 and the valve plate 207. The space prevents the pistons 206 from interfering with the valve plate 207.
However, when the drive shaft 204 is moved rearward such that the top dead center of each piston 206 is moved by a distance greater than the axial dimension of the space between the top dead center and the valve plate 207, the pistons 206 collide with the valve plate 207. The collision generates noise and vibration and damages the piston 206 and the valve plate 207. In other words, the life of the compressor is shortened.
When the drive shaft 204 is displaced rearward, the armature 220, which is secured to the drive shaft 204, is also moved rearward, or brought closer to the pulley 221, which is coupled to the engine Eg. If the core 222 is de-excited in this state, the armature 220 may not be moved to a normal disconnection position but may contact the pulley 221. This creates noise and heat in the clutch 219 and reduces the life of the clutch 219.
Further, when the drive shaft 204 is moved rearward, the lip seal 203a is displaced from a contact area, or predetermined position relative to the drive shaft 204. Sludge is often adhered to the drive shaft 204 at locations other than the location of the contact area. Thus, the lip seal 203a may be moved onto sludge, which degrades the lip seal 203a and causes gas to leak from the crank chamber 203.
If the force of the spring 213 acting on the shaft 204 is increased to prevent axial movement of the drive shaft 204, an increased force acts on the thrust bearings 205b, 212. Therefore, the bearings 205b, 212 are worn in a relatively short time, which reduces the life of the compressor. Also, the force required for rotating the drive shaft 204 is increased, which lowers the compression efficiency of the compressor.
Accordingly, it is an objective of the present invention to provide a variable displacement compressor that prevents noise, vibration, gas leak and guarantees a secure disconnection of a clutch when the cam plate inclination is suddenly decreased based on external information.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a variable displacement compressor is provided. The compressor includes a crank chamber, a drive shaft rotatably supported by and extending through the crank chamber and a cam plate supported by the drive shaft in the crank chamber. The inclination of the cam plate is changeable. The compressor also includes a piston coupled to the cam plate. The piston is reciprocated by a stroke in accordance with the inclination of the cam plate. The compressor further includes a valve plate, a control valve, a limit member and a stopper. The valve plate is located at the opposite side of the piston from the crank chamber. The control valve controls the difference between the pressure in the crank chamber and the pressure at the valve plate, which act on the piston, thereby changing the inclination of the cam plate to control the displacement of the compressor. The limit member is attached to the drive shaft and is located next to the cam plate. The limit member defines the minimum inclination of the cam plate. The stopper prevents the drive shaft from moving toward the valve plate by a significant amount when the cam plate contacts the limit member. The stopper includes the valve plate and rigid material lying between the valve plate and the drive shaft.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.